Your AODD pump runs, the downstream valve closes, and the pump goes quiet. That is deadhead behaviour — and it is supposed to happen. What most operators do not realise is that how your pump behaves at deadhead tells you more about your system than almost any other operating condition.
Understanding the mechanism, and what deviations from it mean, can save you from misdiagnosed failures and poorly sized replacement equipment.
What Actually Happens at Deadhead
An AODD pump works by using compressed air to push a diaphragm, which displaces fluid through check valves. The pilot valve — a small internal spool valve — switches the air between the two chambers alternately, driving the diaphragm back and forth.
When the downstream valve closes, back pressure builds in the discharge line. The diaphragm now has to push against that back pressure on every stroke. As back pressure rises toward the air supply pressure, the diaphragm cannot complete its stroke — it reaches equilibrium before it travels far enough to shift the pilot valve. The pilot valve stops switching. The pump stalls.
This is not a fault. It is the pump’s self-limiting behaviour, and it is one of the key advantages of AODD technology over centrifugal pumps, which will deadhead and overheat. A correctly functioning AODD pump at deadhead holds pressure equal to the air supply pressure and consumes almost no air.
The Stall Pressure Is Your Air Supply Pressure
This is the point that has the most practical consequence for system design: the maximum pressure an AODD pump can generate is determined by the air supply, not by the pump size or model.
If your air supply is 6 bar, the pump will stall at approximately 6 bar differential. It cannot generate more than that regardless of how long it runs. This means your downstream piping, valves, fittings, and instruments must be rated for at least that pressure. If someone increases the air supply to 7 bar to “get more flow,” the stall pressure increases to 7 bar too — and anything downstream now sees that pressure whenever a valve closes.
Specifying pressure relief valves, selecting pipe pressure ratings, and sizing pulsation dampeners all depend on knowing the stall pressure. The air supply regulator setting is the number you need.
What a Clean Stall Looks Like
A pump in good condition with intact diaphragms and seating check valves will stall cleanly. You will hear the strokes slow, then stop. The discharge pressure gauge — if fitted — will stabilise at or near the air supply pressure. Air consumption will drop to near zero. No cycling, no chattering.
If you open the downstream valve again, the pump resumes immediately. The pilot valve has not failed — it was simply waiting for the differential pressure to drop enough to allow the diaphragm to complete its stroke.
When the Pump Does Not Stall Cleanly
Deviations from clean deadhead behaviour are diagnostic. They point to specific failure modes.
The pump continues to cycle at low stroke rate even at full deadhead. This almost always indicates check valve leakage. Fluid is slipping past one or more ball seats on every stroke, so the back pressure never fully holds, and the diaphragm keeps partially completing its stroke. The pump cannot build full system pressure, and it continues to consume air without doing useful work. Inspect the balls and seats — worn seats, swollen or deformed balls, or particle contamination under the ball are the usual causes.
The pump stalls correctly, but pressure bleeds back slowly after stopping. If the discharge pressure gauge drops after the pump stalls — before any downstream valve opens — fluid is migrating back through the pump. Again, check valve integrity is the first place to look. If the check valves are sound, the diaphragm itself may have a pinhole or early-stage crack allowing fluid to bypass from the wet side to the air side. A small amount of fluid weeping from the exhaust muffler during this bleed-back test confirms diaphragm damage.
The pump chatters rapidly without building pressure. This typically indicates an air distribution problem — a worn or damaged pilot valve spool, or an obstruction in the air passages. The pump is switching air rapidly between chambers but the diaphragm is not completing a full stroke in either direction, so no net pressure builds. Replace the pilot valve assembly and check the centre manifold for debris.
Deadhead as a Commissioning Check
A controlled deadhead test at commissioning is one of the most useful things you can do before a pump enters service. Close the downstream isolation valve, set the air pressure to your intended operating supply, and allow the pump to run until it stalls. Record the time to stall, the stall pressure, and confirm air consumption drops to near zero.
This baseline gives you something to compare against in future. If the pump later fails to reach the same stall pressure, or takes longer to stall, or does not stall at all, the change in behaviour points directly to where the problem has developed.
The System Design Implication You Cannot Ignore
AODD pumps are often specified for their ability to run deadheaded without damage — and that capability is real. But “able to run deadheaded” does not mean “designed to run deadheaded routinely.” A pump that deadheads frequently because of poor process control, oversized pipe work with no flow regulation, or intermittent batch operation is accumulating air valve wear and thermal stress in the compressed air system that shortens service intervals.
If your application involves regular deadhead conditions — as in a transfer pump that fills a tank and then waits — consider fitting an air-operated solenoid or a flow switch to shut the air supply when flow stops. The pump lives longer, compressed air consumption drops, and you eliminate the risk of sustained elevated pressure on downstream fittings and seals.
The deadhead stall is a feature. Use it as a diagnostic tool, account for its pressure implications in your system design, and do not mistake clean stall behaviour for a reason to ignore it.
If your AODD pump is not stalling cleanly — or if you are seeing pressure behaviour that does not match what is described here — contact the Autoflo team at info@autoflotechnology.com. We can help you work through the diagnostics.